Riak is a document oriented database written in Erlang that is highly fault tolerant and based on Dynamo and the CAP theorem. It uses a similar data model to MongoDB by storing semi-structured data as documents but achieves high availability through quorum writes rather than MongoDB's in-place writes. Key features of Riak include configurable replica counts for reads and writes (N/R/W), masterless replication, and an integer keyspace that allows any node to service requests. Riak search also provides distributed, fault tolerant full-text search capabilities.